The present invention relates generally to integrated circuit (IC) designs, and more particularly to a memory device with an asymmetric layout structure.
FIG. 1 schematically illustrates a typical static random access memory (SRAM) cell 100 that is often used in memory devices for electronic products, such as cellular phones, digital cameras, personal digital assistants, and personal computers. The cell 100 includes two cross-coupled inverters 102 and 104. The inverter 102 is comprised of a pull-up p-type metal-oxide-semiconductor (PMOS) transistor 106 and a pull-down n-type metal-oxide-semiconductor (NMOS) transistor 108. The inverter 104 is comprised of a pull-up PMOS transistor 110 and a pull-down NMOS transistor 112. The sources of the PMOS transistors 106 and 110 are coupled to a power supply Vcc. The sources of the NMOS transistors 108 and 112 are coupled to ground or a complementary power supply through Vss. The gates of PMOS transistor 106 and NMOS transistors 108 are connected together at a node 114, which is further connected to the drains of PMOS transistor 110 and NMOS transistor 112. The gates of PMOS transistor 110 and NMOS transistor 112 are connected together at node 116, which is further connected to the drains of PMOS transistor 106 and NMOS transistor 108. A first pass gate transistor 118 is coupled between the node 114 and a bit line BL, and a second pass gate transistor 120 is coupled between the node 116 and a complementary bit line BLB. The cross-coupled first and second inverters 102 and 104 function as a latch that stores a value and its complement at the nodes 114 and 116, respectively.
FIG. 2 illustrates a layout structure 200 of the SRAM cell shown in FIG. 1. A first n-type doped region 202 and a second n-type doped region 208 are formed on p-wells in a semiconductor substrate. A first p-type doped region 204 and a second p-type region 206 are formed on the semiconductor substrate. A gate conductive layer 210 is formed above the doped region 202 and across it along its transverse direction. A gate conductive layer 212 is formed above the doped regions 202 and 204 and across them along their transverse directions. Similarly, a gate conductive layer 216 is formed above the doped region 208 and across it along its transverse direction. A gate conductive layer 214 is formed above the doped regions 208 and 206 and across them along their traverse directions. The gate conductive layer 210 and the doped region 202 thereunder function as a pass gate transistor PG. The gate conductive layer 212 and the doped regions 202 and 204 thereunder function as a pull-down transistor PD and a pull-up transistor PU, respectively. A contact 240 for the source of the pull-down transistor PD, a contact 222 for the drain of the pass gate transistor PG, and a contact 220 for the source of the pass gate transistor PG are formed on the doped region 202, and separated by the gate conductive layers 210 and 212.
As the semiconductor processing technology advances, the scale of the layout structure 200 for the SRAM cell 100 becomes increasingly small. As a result, the gate conducive layer 210 and the contact 220 for the source terminal of the PG transistor become increasingly close. This causes the gate conductive layer 210 and the contact 220 to be particularly susceptible to a birding effect, due to insufficient clearance therebetween. Such bridging effect is likely to induce leakage current. For example, due to process variations, the gate conductive layer 210 may down shift toward the contact 220. As a result, the leakage current would occur due to the closeness between the gate conductive layer 210 and the contact 220.
As such, what is needed is a memory device with improved layout structure that helps reduce the leakage current between the gate and source of the pass gate transistor PG.